1. Field of the Invention
The present invention relates to an optically pumped disk-type solid state laser oscillator that oscillates a laser beam by a light energy supplied from the outside and an optically pumped disk-type solid state laser system.
2. Description of Related Art
In a laser beam output by the solid state laser system, exhausting of heat generated in the interior of the solid state laser medium (laser gain material) poses a large problem. FIG. 1 shows an outline configuration of a conventional rod-type solid state laser system. In the rod-type solid state laser system shown in FIG. 1, when excitation light 2 is irradiated on a rod-type laser gain material 1 and the rod-type laser gain material 1 outputs a laser beam 3 in its axial direction, a thermal gradient generated in the interior of the rod-type laser gain material 1 causes a quality degradation of the laser beam 3, such as an output power decrease of the outputted laser beam 3, and breakage of the rod-type laser gain material 1 self.
Because of this problem, as a method for decreasing the temperature gradient in the rod-type laser gain material, a disk-type solid state laser system in which the laser gain material is formed to be a thin film (made into a disk) in its axial direction was devised. Formation of the laser gain material into a disk shape makes it possible to enlarge a light receiving plane of the excitation light irradiated from the outside and the whole surface of the disk is uniformly cooled, which enables the temperature gradient in the laser gain material to be suppressed. The disk-type solid state laser system is broadly divided into a transmission type system (FIG. 2) and a reflection type system (FIG. 3) in terms of a method for amplifying laser light.
As shown in FIG. 2, in a transmission-type disk-type solid state laser system, in order to cancel a temperature rise and the temperature gradient inside the laser gain material 4 resulting from irradiation of the excitation light 2 on the thin film disk-type laser gain material (transmission type) 4 from the outside, usually, the heat in the interior of the gain material is exhausted (5) to the outside by cooling parallel planes (disk planes) of the thin film disk-type laser gain material (transmission type) 4 from both sides thereof with a cooling medium. However, the laser beam 3 generated inside the thin film disk-type laser gain material (transmission type) 4 also passes through the cooling medium simultaneously. Because of this, distortion occurs in the laser beam 3 passing through the cooling medium and an operational limitation (for example, the cooling medium supply is stopped during laser oscillation) arises.
On the other hand, in the reflection-type disk-type solid state laser system shown in FIG. 3, a reflecting film 6a is joined together to one of parallel planes of a thin film disk-type laser gain material (reflection type) 6. By this arrangement, when the excitation light 2 is irradiated on a parallel plane of the thin film disk-type laser gain material (reflection type) 6 to which the reflecting film 6a is not joined, the laser beam 3 is outputted from a surface of the parallel plane. By providing a cooling mechanism that exhausts heat from the whole parallel plane to which the reflecting film 6a is joined, the heat 5 accumulated in the interior of the thin film disk-type laser gain material (reflection type) 6 can be exhausted to the outside effectively.
In the present embodiment, it becomes possible for the resonator to output the laser beam 3 from the parallel plane to which the reflecting film 6a is not joined, without being affected by the cooling mechanism. Thus, in the reflection-type disk-type solid state laser system, as compared to the transmission-type, since the excitation light 2 is irradiated on the plane to which the reflecting film 6a of the thin film disk-type laser gain material (reflection type) 6 is not joined and is reflected by an inner surface of the reflecting film 6a that is joined on an opposite plane, a distance of an optical path of the excitation light 2 that propagates inside the thin film disk-type laser gain material (reflection type) 6 is increased by a thickness of the thin film disk-type laser gain material (reflection type) 6, and the laser beam 3 is amplified additionally by the amount of the optical path increase effected by the excitation light 2. Thus, even if the laser has the thin film disk-type laser gain material of the identical shape, the refection type laser has an advantage of capability of outputting the laser beam 3 with a large amplification degree as compared to that of the transmission type.
However, also in the case of the reflection-type disk-type solid state laser system, the excitation light 2 can enter only one parallel plane of the thin film disk-type laser gain material, and there remains an effect of the temperature gradient inside the thin film disk-type laser gain material. Moreover, since both systems use stimulated emission in a disk thickness direction (axial direction) that delivers a small gain, not using laser oscillation based on stimulated emission in a disk radial direction that delivers a large gain, it is difficult for a single disk to output the laser beam 3 with a large gain.
On the other hand, it is difficult to use the laser oscillation based on the stimulated emission in the disk radial direction having the large gain because of the shape of the thin film disk-type laser gain material. Moreover, the stimulated emission in the disk radial direction having the large gain affects stimulated emission in the disk thickness direction that contributes to actual laser oscillation, lowering the efficiency of stimulated emission in the disk thickness direction.
A report as shown below has been made in relation to the above-mentioned technique.
“Diode pumped solid state disk laser and method for generating uniform laser gain” disclosed in Japanese Laid-Open Patent Application JP-P2004-349701A proposes an amplification module for sold-state laser that substantially includes a disk having two substantially parallel surfaces and an outer circumferential portion and including an optical gain material, and a plurality of diode bars that are arranged to encircle the outer circumferential portion of the disk and are configured to give an optical pump irradiation to an optical gain material, wherein each of the plurality of diode bars is aligned spatially with respect to the disk so as to generate a substantially uniform gain across the optical gain material.
Furthermore, a “side-face pumped active mirror solid state laser for high output” disclosed in Japanese Laid-Open Patent Application JP-P2004-521490A is a solid sate laser module for amplifying laser irradiation that substantially includes:
a substrate including a surface on which a plurality channels are formed; and a laser grain medium having a front surface, a back surface, and a peripheral edge surface, the back surface contacting the surface of the substrate, wherein the solid state laser module further includes a non-doped optical medium that is attached to the peripheral edge, and an optical pumping irradiation source, the source directs optical pumping irradiation to the non-doped optical medium, the non-doped optical medium transfers the optical pumping irradiation to the laser gain medium; and the channel is maintained at a low pressure so that a difference of pressure is formed between the front surface of the laser gain medium and the back surface of the laser gain medium and the laser gain medium is fixed on the substrate to get along with it.
Still furthermore, “Solid state Laser Engineering 5th Edition” (W. Koechner, Springer, 1999, pp 447-463) illustrates, as an “active mirror amplifier,” a reflection-type disk-type solid state laser system is configured such that a reflecting film is coated on one of parallel planes of a thin film disk-type laser medium and excitation light is irradiated on the same disk plane as is coated with the reflecting film, and illustrates, as a “disk amplifier,” a laser system such that gain is improved by using a plurality of disk-type solid state laser mediums arranged serially in a transmission-type disk-type solid state laser system described in this specification.